1. Field of the Invention
The present invention relates to a voltage conversion device and a computer-readable recording medium having a program recorded thereon for a computer to execute control of voltage conversion by a voltage conversion device.
2. Description of the Background Art
Hybrid vehicles and electric vehicles have recently been of great interest as environment-friendly vehicles. A hybrid vehicle has, as its power sources, a DC (direct current) power supply, an inverter and a motor driven by the inverter in addition to a conventional engine. More specifically, the engine is driven to provide a power source and a DC voltage from the DC power supply is converted by the inverter into an AC (alternating current) voltage to be used for rotating the motor and thereby providing a power source.
An electric vehicle refers to a vehicle that has, as its power sources, a DC power supply, an inverter and a motor driven by the inverter.
Regarding such a hybrid vehicle or electric vehicle, a configuration has also been studied with which a DC voltage from the DC power supply is stepped up by a voltage step-up converter and the stepped up DC voltage is supplied to the inverter that drives motor (for example, see Japanese Patent Laying-Open Nos. 08-214592 and 2005-051895).
The voltage step-up converter is comprised of two NPN transistors connected in series between a power supply line and a ground line of the inverter and a reactor having one end connected to an intermediate point between the two NPN transistors and the other end connected to a power supply line of the power supply.
The voltage step-up converter turns on/off the NPN transistor connected to the power supply line (upper arm) and the NPN transistor connected to the ground line (lower arm) at a predetermined duty ratio so as to step up a DC voltage from the power supply and supply the stepped up voltage to the inverter while stepping down a DC voltage from the inverter to supply the stepped-down voltage to the power supply.
Since the upper arm and the lower arm that are components of the voltage step-up converter are connected in series between the power supply line and the ground line, the upper arm and the lower arm have to be prevented from being simultaneously ON. Therefore, to a control signal for controlling switching of the upper arm and the lower arm, a dead time is provided for preventing the upper arm and the lower arm from being simultaneously ON.
FIG. 28 is a timing chart of control signals controlling the upper arm and the lower arm.
Referring to FIG. 28, the upper arm and the lower arm are turned on/off at a predetermined duty ratio in each control period T. The lower arm is kept ON until timing t1 while the upper arm is kept OFF until timing t1. If the upper arm is thereafter turned on and the lower arm is thereafter turned off at timing t1, the upper arm and the lower arm could be ON at the same time. Therefore, the lower arm is turned off at timing t1 and the upper arm is turned on at timing t2 at which a certain dead time has passed since timing t1.
However, if a voltage command value of the voltage step-up converter is considerably close to a power supply voltage, the on-duty of the upper arm (referring to the period during which the upper arm is kept ON) is fairly high, for example, 0.98. In such a case, a part of the on-duty 0.98 is taken or occupied by the dead time, and thus the time during which the upper arm should be kept ON cannot be ensured. In other words, in a region of the on-duty that is fairly close to 1.0, there arises a dead zone where any on-duty cannot be ensured as it is due to the dead time.
FIGS. 29A and 29B are timing charts respectively of the voltage and on-duty of the upper arm.
Referring to FIG. 29A, supposing that an operation of stepping up power supply voltage Vb is started at timing t0, the voltage command value increases from power supply voltage Vb. In the period from timing t0 to timing t1, the voltage command value is very close to power supply voltage Vb. Therefore, the on-duty of the upper arm that is calculated based on the voltage command value is partially occupied by the dead time of the upper arm and thus the original on-duty cannot be ensured. As a result, the on-duty of the upper arm that is in the range for example of 1.0 to 0.95 cannot be controlled linearly and thus oscillates (see FIG. 29B). Accordingly, the output voltage of the step-up converter also oscillates (see FIG. 29A).
As the on-duty of the upper arm that is calculated based on the voltage command value reaches for example 0.95, the on-duty is not partially occupied by the dead time and the on-duty can be controlled linearly.
As seen from the above, when the voltage command value is in a region fairly close to power supply voltage Vb, the on-duty of the upper arm is partially occupied by the dead time, the output voltage of the voltage step-up converter oscillates and the DC current from the power supply also oscillates. As a result, the power supply could be broken.
In addition, in a period in which a voltage step-up operation is performed, when the on-duty of the upper arm is in the above-described dead zone, the original on-duty cannot be ensured while the on-duty suddenly changes to 1.0 at a timing at which the voltage step-up operation is stopped. At this time, as the on-duty suddenly changes, the output voltage of the voltage step-up converter also suddenly decreases to power supply voltage Vb. Accordingly, the DC current from the power supply suddenly increases. As a result, the power supply is deteriorated in performance because the excessively large DC current flows, and accordingly shorten in lifetime.